[Wylug-discuss] Help needed with 'failed' Linux software RAID 10

[John Hodrien]

On Sun, 20 Jun 2010, Dave Fisher wrote:

> On 20 June 2010 17:56, John Hodrien <J.H.Hodrien at leeds.ac.uk> wrote:
>> So what *has* triggered the failures of your RAID sets?
>
> An as yet unidentified problem with one or more of motherboard, cpu or
> memory.  I suspect the motherboard. I originally suspected the PSU,
> but a fully tested replacement was no help.

If you can't trust the motherboard, RAM, or CPU, then it doesn't matter what
storage technology you use, you're in a fairly grim position unfortunately.
You can get horrible problems with hardware RAID when the controller RAM goes
bad.  It's normally caught by ECC, but apparently not always.

>> If you're not happy with managing the RAID yourself under linux, have you
>> considered buying a SOHO NAS box?  A 7 bay NAS with 2Tb disks in RAID6 would
>> get you 10Tb usable, with cover for any two disks failing.  I'm personally
>> wouldn't be concerned about the write performance hit.  You'd probably pay
>> about 2k, and it'd be a straightforward appliance, offering iSCSI or NFS/CIFS
>> to taste.  Accessed over gigabit, you'd be limited to <100Mbytes/sec, but I'm
>> not sure that'd be an issue for what you need.
>
> Yeah, I'm seriously considering that option, but 2k is well out of my
> league at present ... the recession (soon to get worse) has already
> badly hit my income.

It's an expensive problem.  10Tb, ideally duplicated, desirably RAIDed, a
portion of which is ideally backed up.  Your cheapest option then probably
becomes 2Tb disks x 5 RAID0 with a pair of identical machines.  You can get
2Tb disks for about £100.  If you'd already got machines that could take the
disks, then you're in a much better position.  Setup a nightly rsync between
the machines.  It's not fully satisfying the ideal, but it's much cheaper.

> I now know a lot more than I ever wanted to about Linux RAID, but
> it'll probably be out of my head by the time another emergency occurs.
>
> Thanks for the tip on write performance.

It's a real issue for optimising databases, but I don't think it's something
you generally need to lose sleep over.  I've got RAID5 arrays that beat RAID0
arrays for write performance.  Good disks, and a decent RAID card, and it's
perfectly good for general use.

Just a quick marker for performance:
5 disk RAID5 set with 300Gbyte SAS disks on a PERC6/i ~250Mbyte/sec sequential
writes, ~450Mbyte/sec sequential reads.  Sequential performance doesn't dive
as much as you'd think switching to cheaper bigger disks.

jh